1. Field of the Invention
The present invention relates to wireless IC devices, and in particular, relates to wireless IC devices using Radio Frequency Identification (RFID) systems enabling data communication using electromagnetic waves in a non-contact manner.
2. Description of the Related Art
Recently, RFID systems enabling information transmission between readers/writers that generate induction fields and RFID tags that store predetermined information on articles in a non-contact manner have been used as systems for controlling articles. Moreover, Japanese Unexamined Patent Application Publication No. 2007-18067 discloses a combination tag including both an RFID tag and a resonant tag.
FIG. 1 illustrates the structure of the combination tag described in Japanese Unexamined Patent Application Publication No. 2007-18067. As shown in FIG. 1, an RFID tag 60 includes an inlet sheet 61 and an overlay sheet 62 that protects surfaces of the inlet sheet 61. The inlet sheet 61 includes a substrate on which an IC chip 64, a first antenna 65, and a second antenna 66 are mounted. The overlay sheet 62 includes a top sheet and a bottom sheet. The first antenna 65 is a substantially spiral coil antenna, and is formed of a conductor electrically connected to two terminals of the IC chip 64. The first antenna 65 generates power for starting a CPU of the IC chip 64, and receives signals sent from an antenna unit of a reader/writer. The second antenna 66 is formed of a conductor as is the first antenna 65. However, the second antenna 66 is electrically insulated from the IC chip 64. The second antenna 66 is a substantially spiral coil antenna whose inductance component L and capacitance component C form an LC resonant circuit.
The second antenna 66 consumes energy by resonating in a near electromagnetic field generated by the antenna unit, resulting in an increase in return loss. The presence of the RFID tag 60 is detected by the reader/writer when the antenna unit detects the return loss.
However, the size of the combination tag described in Japanese Unexamined Patent Application Publication No. 2007-18067 is disadvantageously large since the second antenna for a resonant tag is disposed adjacent to the first antenna for the RFID tag. Moreover, each of the antennas resonates at a frequency corresponding to the inductance determined by the length of the electrode and the stray capacitance generated among wires of the substantially spiral electrode. When an electromagnetic field is radiated to the second antenna, the first antenna operating as an RFID tag in response to the electromagnetic field, the RFID tag and the resonant tag can resonate at the same time in the case where the resonant frequencies of the first antenna and the second antenna are close to each other. At this moment, the electromagnetic field generated at the resonant tag can disturb the electromagnetic field generated at the RFID tag, and can prevent the operation of the antenna as the RFID tag.